The present invention relates to a capacitor element for a power capacitor, including at least one elongated film layer of dielectric material, and an electrode layer of metal material arranged on each flat side of the film layer, which two electrode layers are divided into at least three metallized areas, separated from each other, in order to form an inner series connection arranged to conduct a load current. The three areas extend in the longitudinal direction of the film layer and at least one area of said at least three areas is divided into segments separated by non-metallized sections, and at least one bridge arranged to electrically connect two of the segments together.
The invention also relates to a power capacitor comprising such a capacitor element and also a metallized tape for such a power capacitor.
In this context power capacitors refers to capacitors for alternating or direct current applications for voltages exceeding 1 kV, preferably at least 5 kV.
In power capacitors it is known to use rolled capacitor elements comprising metallized tapes formed from sandwiched films of dielectric material and electrode layers of metal material. It is also known to divide the electrode layer into electrically separated, parallel metallized areas extending in the longitudinal direction of the films, in order to achieve series-connected part-capacitors between the end surfaces of the roll, known as xe2x80x9cinner series connectionxe2x80x9d. It is further more known to divide one or more of the metallized areas into segments separated by non-metallized sections, and also bridges arranged to electrically connect the segments together. Segmentation increases the equivalent surface resistivity of the metallized area, which has been found to benefit the so called xe2x80x9cself-healingxe2x80x9d capacity of the capacitor element. This means that in the event of an electric puncture in a film layer, the electrode material nearest the fault point is vaporized due to a powerful, but brief, discharge current endeavouring to pass the short circuit. The electric strength in the area is restored when the electrode material nearest the fault point has vaporized, and the capacitor element has thus self-healed. As a result of the lost electrode area, every self-healing process produces a slight decrease in the capacitance of the capacitor element. If a self-healing process occurs in a segment, the bridges ensure that sufficient energy can be transferred from adjacent elements to enable efficient vaporization of the electrode material around the fault point. If a serious fault occurs, e.g. if the puncture passes through several film layers, a strong current surge occurs through the bridges connected to the defective segment. If the current surge is sufficiently strong the bridges themselves may be vaporized, in which case the faulty segment is isolated. The lose in capacitance is in this case greater than at the self-healing process.
A segmented, metallized tape for a capacitor element of the above type is described, for instance, in the document GB 2 298 962A in connection with FIG. 4. The tape described consists of a film on which two segmented, metallized areas are arranged. Each segment is connected by bridges to two of its neighbouring segments. The metallized film is in this case intended for a capacitor element with one inner series connection.
During operation the rolled capacitor element is electrically connected at both its end surfaces and a current, in the following termed load current, flows continuously between these surfaces and causes Joule losses in the capacitor element. The load current follows the path of least resistance between the end surfaces. In the case of alternating current the alternating current itself is responsible for a considerable proportion of the Joule losses and in the case of direct current, ripple currents are responsible for most of the Joule losses. One problem with capacitor elements comprising known metallized tape of the type described above is that Joule losses occur in the bridges during normal operation. This is particularly so in applications involving high current. The generation of heat in known capacitor elements with metallized tape of the above type may be so great that it determines the dimensions of their design.
One object of the present invention is to eliminate the above-mentioned problems and provide a capacitor element comprising a segmented, metallized area where two adjacent segments are electrically connected by means of a bridge in accordance with a new principle.
The capacitor element and the power capacitor in accordance with the invention provide the bridge so arranged that the load current in the area divided into segments appears primariry in segments.
The metallized tape in accordance with the invention provides that at least one of the bridges is arranged in a portion of the non-metallized section that extends perpendicularly or substantially perpendicularly to the long sides of the dielectric film.
In accordance with the invention the bridge is arranged so that the load current appearing in the capacitor element does not pass through this to any great extent. However, the bridge is arranged so that a certain current exchange is permitted between neighbouring segments, e.g. at self-healing processes or in order to equalize small differences in potential. Contrary to the load current, the currents at these current exchanges are so low or of such short duration that they do not cause any great Joule losses.
In accordance with one embodiment of the invention the bridges are arranged in parts of non-metallized sections extending perpendicularly or substantially perpendicularly to the connection surfaces of the capacitor element. Since the load current strives to follow as xe2x80x9cstraightxe2x80x9d a current path as possible, i.e. strives to follow an electric circuit extending perpendicularly to the connection surfaces of the capacitor elements, forming the shortest path between the connection surfaces, a bridge that connects together two segments perpendicularly to this electric circuit will not be included in the electric circuit of the load current. This embodiment of the invention also contributes to a high resistance perpendicular to the direction of the load current, which is beneficial.
In accordance with another embodiment of the invention the segments connected together by the bridge are uniform.
In accordance with one embodiment of the power capacitor according to the invention, the power capacitor comprises a plurality of capacitor elements, that have substantially circular-cylindrical shape, are arranged close together so that their axial directions coincide, and are connected to each other so that they form a series-connected capacitor stack. In such a power capacitor for high voltage the technique of using inner series-connections in the capacitor elements is an obvious advantage since the number of series-connected capacitor elements can be reduced. The technique is particularly advantageous together with the technique mentioned above for self-healing. Since successful self-healing requires particularly thin metal coating and the currents flowing through the metal generate active power dissipation (heat), thinner layers result in higher losses. One way of reducing the losses without compromising the requirement for a thin metal coating is to choose a shape for the metallized film, and thus a shape for the capacitor element, such that the dimension of the metal coating perpendicular to the rolling direction is decreased and the length of the roll is increased. Unless internal series-connection is used, the consequence of this will be that the cylindrical capacitor elements acquire a relatively little height in relation to their diameter. Series-connecting many such elements, which is required for high voltage, becomes detrimental from the cost point of view. With inner series connections, therefore, several series-connected part-capacitors can automatically be built into a cylindrical capacitor element with an optimal relation between height and diameter, from the manufacturing aspect, and with good self-healing properties.